Thin film transistor, method for manufacturing the same, and display device comprising the same

ABSTRACT

A thin film transistor, a method of manufacturing the thin film transistor, and a display device including the thin film transistor are provided. The thin film transistor comprises an oxide semiconductor layer, a gate electrode, a source electrode and a drain electrode formed on a substrate in a coplanar configuration. A first conductive member is in direct contact with the oxide semiconductor layer and in direct contact with the source electrode. A second conductive member is in direct contact with the oxide semiconductor layer and in direct contact with the drain electrode. The first conductive member and the second conductive member are arranged to decrease resistance between a channel region of the oxide semiconductor layer and the source and drain electrodes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/095,617 filed on Dec. 3, 2013, which claims priority to and thebenefit of Korean Patent Application No. 10-2012-0144970, filed on Dec.12, 2012, and also claims priority to and the benefit of Korean PatentApplication No. 10-2013-0092414, filed on Aug. 5, 2013, all of which areincorporated herein by reference in their entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to a thin film transistor allowing anelement characteristic of the thin film transistor having a coplanarstructure using an oxide semiconductor to be enhanced, a method ofmanufacturing the thin film transistor, and a display device includingthe thin film transistor.

2. Discussion of Related Art

As much attention has been focused recently on information displays andneeds for a portable electronic device has been increasing, research andcommercialization on a lightweight and thin film type display device anda large-sized high-resolution display device are widely made. Inparticular, among these various display devices, research on a liquidcrystal display (LCD) and an organic light emitting display (OLED) iswidely made.

In the LCD and the OLED, a thin film transistor (TFT) is used as aswitching element and/or a driving element. The thin film transistor isclassified as a thin film transistor using amorphous silicon, a thinfilm transistor using polysilicon, or a thin film transistor using anoxide semiconductor depending on a material used as an active layer. Inthe case of the thin film transistor using the polycrystalline silicon,a process of implanting ions is carried out to adjust a resistance ofthe active layer. An additional mask for defining an ion implantationregion may be used, and the ion implantation process is added, therebycausing a disadvantage in terms of process. On the other hand, in thecase of the thin film transistor using the oxide semiconductor, theelectron mobility increases as compared to that of the thin filmtransistor using the amorphous silicon, an amount of the leakage currentis significantly lower than that of the thin film transistor using theamorphous silicon and the thin film transistor using the polycrystallinesilicon, and a high reliability test condition is satisfied. Inaddition, the thin film transistor using the oxide semiconductor canadvantageously ensure that a distribution of threshold voltages isuniform as compared to the thin film transistor using thepolycrystalline silicon.

The thin film transistor using the oxide semiconductor may be classifiedas a thin film transistor having an inverted-staggered structure or athin film transistor having a coplanar structure depending on positionsof an active layer, a gate electrode, a source electrode, and a drainelectrode. Since the thin film transistor having the inverted-staggeredstructure has a high parasitic capacitance between the gate electrodeand the source and drain electrodes, it is difficult to apply the thinfilm transistor having the inverted-staggered structure to ahigh-resolution display.

Inventors of the present invention have recognized that a highresistance occurred due to an interval of several micrometers between aportion in which the active layer and the source and drain electrodesare in contact with each other and a channel region of the active layerin the thin film transistor having the coplanar structure. To addressthis problem, the inventors have made the thin film transistor havingthe improved coplanar structure.

SUMMARY OF THE INVENTION

Accordingly, an aspect of the present disclosure relates an improvedthin film transistor (TFT) configured with one or more of conductivemembers for reducing the electrical resistance between the channelregion of the oxide semiconductor layer and the electrodes (e.g., sourceand drain electrodes) of the TFT.

In one embodiment, a TFT comprises an oxide semiconductor layer, a gateelectrode, a source electrode and a drain electrode formed on asubstrate in a coplanar configuration. The TFT further comprises a firstconductive member and a second conductive member that are in directcontact with the oxide semiconductor layer. The first conductive memberis in direct contact with the source electrode on the source regions ofthe oxide semiconductor layer whereas the second conductive member is indirect contact with the drain electrode on the drain region of the oxidesemiconductor layer. The first conductive member and the secondconductive member are arranged to decrease resistance between a channelregion of the oxide semiconductor layer and the source and drainelectrodes.

In one embodiment, an insulating member may be formed on the channelregion of the oxide semiconductor layer and between the first and secondconductive members. In this configuration, the first conductive member,the insulating member and the second conductive member forms a singlelayer disposed on the oxide semiconductor layer. In an embodiment, thesurface characteristic of the channel region of the oxide semiconductorlayer is different from the surface characteristic of the source anddrain regions of the oxide semiconductor layer. Also, in an embodiment,the single layer is formed of an oxidizable conductive material, and aportion of the single layer is oxidized so that the oxidized portionforms the insulating member. The first and second conductive members arenon-oxidized portions of the single layer, which are positioned at theopposite ends of the oxidized portion. The length of the insulatingmember in the single layer may vary. In one embodiment, thecross-sectional length of the insulating member is equal to or greaterthan the cross-sectional length of the gate electrode. Accordingly, theinsulating member may extend beyond the overlapping gate electrode suchthat the gate electrode is arranged not to overlap with the first andthe second conductive members.

In one embodiment, the source electrode is in contact with the firstconductive member at a first contact area and the first conductivemember extends away from the first contact area towards the gateelectrode. The drain electrode is in contact with the second conductivemember at a second contact area and the second conductive layer extendsaway from the second contact member towards the gate electrode. In anembodiment, the closest end of the first conductive member is spacedapart from the channel region of the oxide semiconductor layer.Similarly, the closest end of the second conductive member to the gateelectrode is spaced apart from the channel region of the oxidesemiconductor layer. In an embodiment, the thickness of the oxidesemiconductor layer in the channel region is less than or equal to thethickness of the oxide semiconductor layer in the source and drainregions. In this configuration, the space between the first and secondconductive members to the channel region of the oxide semiconductorlayer may be formed by the differences in the heights of the source anddrain regions to the channel region of the oxide semiconductor layer. Inan embodiment, the closest end of the first conductive member to thegate electrode is vertically aligned with the first end of the channelregion whereas the closest end of the second conductive member to thegate electrode is vertically aligned with the opposite end of thechannel region of the oxide semiconductor layer. In an embodiment, thedistance between the first and second conducive members is equal to orgreater than a length of the gate electrode.

In another embodiment, the first conductive member is in contact withone end side surface of the oxide semiconductor layer and the secondconductive member is in contact with an opposing end side surface of theoxide semiconductor layer. The first and the second conductive membersdisposed at the opposing ends of the oxide semiconductor such that thefirst conductive member, the oxide semiconductor and the secondconductive member are disposed in a same plane. In one embodiment, thefirst and the second conductive members have the same thickness as theoxide semiconductor layer. In other embodiment, the thickness of thefirst and the second conductive members may be less than the thicknessof the oxide semiconductor layer depending on the desired distancebetween the gate electrode and the conductive members. Also, in anembodiment, the distance between the first and second conductive membersmay be equal to or greater than the length of the gate electrode suchthat the each end of the first and second conductive members towards theoxide semiconductor is vertically aligned with the opposing ends of thegate electrode.

Another aspect of the present disclosure relates to an oxidesemiconductor based thin film transistor with an auxiliary memberdisposed on the oxide semiconductor layer for reducing the electricalresistance between the channel region of the oxide semiconductor layerand the electrodes (e.g., source and drain electrodes) of the TFT.

In one embodiment, a TFT comprises an oxide semiconductor layer, a gateelectrode, a source electrode, and a drain electrode formed on thesubstrate in a coplanar transistor configuration. The TFT furtherincludes an auxiliary member disposed directly on the oxidesemiconductor layer. The auxiliary member includes an oxidized portionin between at least a first conductive portion and a second conductiveportion. The first conductive portion is in contact with the sourceelectrode whereas the second conductive portion is in contact with thedrain electrode. The oxidized portion disposed between the firstconductive portion and the second conductive portion has lowerelectrical conductivity than that of the first and second conductiveportions. In an embodiment, at least some portion of the oxidizedportion of the auxiliary member is configured to overlap with the gateelectrode. In an embodiment, the length of the oxidized portion in theauxiliary member is equal to or greater than the length of the gateelectrode. The thickness of the auxiliary member may be from about 30 Åto about 100 Å.

An aspect of the present disclosure also relates to a display deviceemploying a coplanar thin film transistor configured with one or moreconductive member for reducing the electrical resistance between thechannel region of the oxide semiconductor layer and the electrodes(e.g., source and drain electrodes) of the TFT.

In one embodiment, a display device comprises a substrate, a coplanarthin film transistor, and a display element. The coplanar thin filmtransistor includes an oxide semiconductor layer, a gate electrode, asource electrode, and a drain electrode formed on the substrate in acoplanar transistor configuration. A first conductive member is indirect contact with the oxide semiconductor layer as well as the sourceelectrode, serving as an extension of the source electrode. Similarly, asecond conductive member is in direct contact with the oxidesemiconductor layer as well as the drain electrode, thereby serving asan extension of the drain electrode. Also included in the display deviceis a display element, which is operatively connected to the coplanarthin film transistor. In an embodiment, the display element is anorganic light emitting element having an anode, a cathode and an organiclight-emitting layer interposed between the anode and the cathode. Theanode is electrically connected to the coplanar thin film transistor. Inanother embodiment, the display element is a liquid crystal displayincluding a pixel electrode, a common electrode and a liquid crystallayer. In this embodiment, the pixel electrode is electrically connectedto the coplanar thin film transistor. In another embodiment, the displayelement comprises a first electrode, a second electrode and an opticalmedium layer interposed between the first and second electrodes. Theoptical medium layer includes a fluid and charged particles dispersed inthe fluid. The charged particles may have a variety of colors andoptical characteristics (e.g., absorbing, reflecting, scattering, etc.).At least one of the first and second electrodes is electricallyconnected to the coplanar thin film transistor to control the movementof the charged particles.

Yet another aspect of the present disclosure relates to a method ofmanufacturing a thin film transistor with one or more conductive memberfor reducing the electrical resistance between the channel region of theoxide semiconductor layer and the electrodes (e.g., source and drainelectrodes) of the TFT.

In an embodiment, the method includes forming an oxide semiconductorlayer on a substrate; forming a first conductive member and a secondconductive member, which are in contact with the oxide semiconductorlayer; and forming a gate electrode, a source electrode and a drainelectrode in a coplanar transistor configuration with respect to theoxide semiconductor layer. The source electrode is formed to directlycontact the first conductive member, and the drain electrode is formedto directly contact the second conductive member.

In one embodiment, the first and second conductive members are formed byoxidizing a portion of an oxidizable conductive layer formed on theoxide semiconductor layer. By oxidizing the portion of the oxidizableconductive layer, an insulating member is formed on a channel region ofthe oxide semiconductor layer, and the insulating member separates thefirst conductive member on a source region and the second conductivemember on a drain region of the oxide semiconductor layer.

In one embodiment, the first and second conductive members are formed byetching a portion of a conductive layer such that the first conductivemember on a source region and the second conductive member on a drainregion of the oxide semiconductor layer, which are spaced apart fromeach other.

In one embodiment, the first and second conductive members are formed byforming the first conductive member in contact with one end side surfaceof the oxide semiconductor layer and forming the second conductivemember in contact with an opposing end side surface of the oxidesemiconductor layer.

Additional features of the invention will be set forth in thedescription, which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent to those of ordinary skill in theart by describing in detail exemplary embodiments thereof with referenceto the accompanying drawings, in which:

FIG. 1A is a plan view illustrating a thin film transistor in accordancewith an embodiment of the present invention;

FIG. 1B is a cross-sectional view illustrating a thin film transistortaken along the line Ib-Ib′ of FIG. 1A;

FIG. 2A is a plan view illustrating a thin film transistor in accordancewith another embodiment of the present invention;

FIG. 2B is a cross-sectional view illustrating a thin film transistortaken along the line IIb-IIb′ of FIG. 2A;

FIG. 3A is a plan view illustrating a thin film transistor in accordancewith yet another embodiment of the present invention;

FIG. 3B is a cross-sectional view illustrating a thin film transistortaken along the line IIIb-IIIb′ of FIG. 3A;

FIGS. 4A to 4C are cross-sectional views illustrating thin filmtransistors in accordance with various embodiments of the presentinvention;

FIG. 5 is a schematic diagram illustrating a display device to whichthin film transistors can be applied in accordance with variousembodiments of the present invention;

FIG. 6 is a flowchart illustrating a method of manufacturing a thin filmtransistor in accordance with an embodiment of the present invention;

FIGS. 7A to 7F are cross-sectional views of respective processesillustrating a method of manufacturing a thin film transistor inaccordance with an embodiment of the present invention;

FIGS. 8A to 8G are cross-sectional views of respective processesillustrating a method of manufacturing a thin film transistor inaccordance with another embodiment of the present invention;

FIGS. 9A to 9D are cross-sectional views of respective processesillustrating a method of manufacturing a thin film transistor inaccordance with yet another embodiment of the present invention; and

FIG. 10 is a flowchart illustrating a method of manufacturing a thinfilm transistor in accordance with another embodiment of the presentinvention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present invention will be described indetail below with reference to the accompanying drawings. While thepresent invention is shown and described in connection with exemplaryembodiments thereof, it will be apparent to those skilled in the artthat various modifications can be made without departing from the scopeof the invention.

An element or layer formed “on” another element or layer includes a casein which an element is directly formed on another element, and a case inwhich an element is formed on another element with an additional elementor layer formed therebetween.

Although the terms first, second, etc. may be used to describe variouselements, it should be understood that these elements are not limited bythese terms. These terms are only used to distinguish one element fromanother. For example, a first element could be termed a second element,and, similarly, a second element could be termed a first element,without departing from the scope of exemplary embodiments.

In this specification, like numbers refer to like elements throughoutthe description of the drawings.

Sizes and thicknesses of respective elements shown in the drawings areshown for the sake of convenience of description only and are notintended to limit the scope of the present invention.

In this specification, an organic light emitting display device with atop emission mode refers to an organic light emitting display device,wherein light emitted from the organic light emitting diode radiatesfrom an upper portion of the organic light emitting display device. Thatis, the organic light emitting display device with a top emission moderefers to an organic light emitting display device, wherein lightemitted from the organic light emitting diode radiates in a direction ofa top surface of a substrate having a thin film transistor formedtherein for driving the organic light emitting display device. In thisspecification, an organic light emitting display device with a bottomemission mode refers to an organic light emitting display device,wherein light emitted from the organic light emitting diode radiatesfrom a lower portion of the organic light emitting display device. Thatis, the organic light emitting display device with a bottom emissionmode refers to an organic light emitting display device, wherein lightemitted from the organic light emitting diode radiates in a direction ofa bottom surface of a substrate having a thin film transistor formedtherein for driving the organic light emitting display device. In thisspecification, an organic light emitting display device with a dualemission mode refers to an organic light emitting display device,wherein light emitted from the organic light emitting diode radiatesfrom upper and lower portions of the organic light emitting displaydevice. In this specification, in the organic light emitting displaydevices with top, bottom, and dual emission modes, a thin filmtransistor, an anode, and a cathode are disposed to optimize aconfiguration of each emission mode, thereby optimally disposing thethin film transistor without interfering with an emission direction of alight emitting element.

In this specification, a flexible display device refers to a displaydevice endowed with flexibility, and may be used to have the samemeaning as a bendable display device, a rollable display device, anunbreakable display device, or a foldable display device. In thisspecification, a flexible organic light emitting display device is oneexample of various flexible display devices.

In this specification, a transparent display device refers to atransparent display device that is at least a part of a screen of adisplay device viewed by a user. In this specification, transparency ofthe transparent display device refers to a degree of transparency atwhich a user at least recognizes an object behind a display device. Inthis specification, the transparent display device includes a displayarea and a non-display area. The display area is an area on which animage is displayed, and the non-display area is an area on which noimage is displayed, such as a bezel area. To maximize transmittance ofthe display area, the transparent display device is configured todispose opaque components, such as a battery, a printed circuit board(PCB), and a metal frame, under the non-display area rather than thedisplay area. In this specification, the transparent display devicerefers to a transparent display device whose transmissivity is, forexample, equal to or greater than at least 20%. In this specification,the term “transmissivity” means a value obtained by dividing anintensity of light, which passes through the transparent display deviceexcept for light which is incident on a transmissive region of thetransparent display device and reflected on the interface betweenrespective layers of the transparent display device, by an intensity ofthe entire incident light.

In this specification, front and rear surfaces of the transparentdisplay device are defined based on light emitted from the transparentdisplay device. In this specification, the front surface of thetransparent display device means a surface on which light from thetransparent display device is emitted, and the rear surface of thetransparent display device means a surface opposite to the surface onwhich the light from the transparent display device is emitted.

The features of various exemplary embodiments of the present inventionmay be partially or entirely bound or combined with each other, and betechnically engaged and driven using various methods as apparent tothose skilled in the art, and the exemplary embodiments may beindependently practiced alone or in combination.

Hereinafter, various exemplary embodiments of the present invention willbe described in further detail with reference to the accompanyingdrawings.

FIG. 1A is a plan view illustrating a thin film transistor in accordancewith an embodiment of the present invention. FIG. 1B is across-sectional view illustrating a thin film transistor taken along theline Ib-Ib′ of FIG. 1A. Referring to FIGS. 1A and 1B, a thin filmtransistor 100 includes a substrate 110, an oxide semiconductor layer120, a conductive layer 130, a first insulating layer 150, a gateelectrode 160, a second insulating layer 170, a source electrode 181,and a drain electrode 182.

The substrate 110 is a member for supporting various elements that maybe formed on the substrate 110. The substrate 110 may be referred to asa support substrate, a lower substrate, a thin film transistor, asupport member, a lower support member, or the like. The substrate 110may be formed of an insulating material such as glass or plastic.However, the substrate is not limited thereto and may be formed ofvarious materials.

The substrate 110 may be formed of various materials depending onvarious applications in which the thin film transistor 100 is used. Forexample, when the thin film transistor 100 is used in a flexible displaydevice, the substrate 110 may be formed of a soft insulating material.In this case, the available soft insulating material may includepolyimide (PI), polyetherimide (PEI), polyethylene terephthalate (PET),polycarbonate (PC), polystyrene (PS), styrene acrylonitrile copolymer(SAN), a silicon-acrylic resin, or the like. In addition, when the thinfilm transistor 100 is used in transparent display device, the substrate110 may be formed of a transparent insulating material.

The oxide semiconductor layer 120 is formed on the substrate 110 as anactive layer of the thin film transistor 100. When the oxidesemiconductor layer 120 is formed as a layer shape, the oxidesemiconductor layer 120 may be referred to as an oxide semiconductorlayer 120. The oxide semiconductor layer 120 may include a first region121, a second region 122, and a third region 123, and the first region121, the second region 122, and the third region 123 may be referred toas a source region, a drain region, and a channel region disposedbetween the source and drain regions, respectively. It is shown in FIGS.1A and 1B that the oxide semiconductor 120 is divided into the channelregion, the source region, and the drain region for convenience ofdescription. However, the oxide semiconductor 120 is not necessarilylimited to the channel region, the source region, and the drain regionshown in FIGS. 1A and 1B.

The oxide semiconductor 120 may be formed of various metal oxides. Forexample, as a constituent material of the oxide semiconductor 120, afour-component metal oxide such as an InSnGaZnO-based material; athree-component metal oxide such as an InGaZnO-based material, anInSnZnO-based material, an InAlZnO-based material, an InHfZnO-basedmaterial, a SnGaZnO-based material, an AlGaZnO-based material, or aSnAlZnO-based material, a two-component metal oxide such as anInZnO-based material, a SnZnO-based material, an AlZnO-based material, aZnMgO-based material, a SnMgO-based material, an InMgO-based material,or an InGaO-based material; an InO-based material; a SnO-based material;a ZnO-based material; or the like may be used. A composition ratio ofeach element included in each of the listed materials of the oxidesemiconductor 120 is not particularly limited and may be variouslyadjusted. In addition, a thickness of the oxide semiconductor 120 is notparticularly limited and may be variously adjusted. The oxidesemiconductor 120 may be formed to have a thickness in a range fromabout 100 Å to about 10000 Å.

The thin film transistor 100 is formed as various shapes by shapes ofvarious elements constituting the thin film transistor 100. For example,the thin film transistor 100 in which the oxide semiconductor 120constituting the active layer is formed as a bar shape as shown in FIG.1A, and the thin film transistor in which the oxide semiconductorconstituting the active layer is formed as U shape are present. Althoughnot shown in the accompanying drawings herein, the thin film transistormay be formed as various shapes by means of shapes of the active layer,the drain electrode, and the source electrode.

The conductive layer 130 is formed on the oxide semiconductor 120 to beelectrically connected to the oxide semiconductor 120. The conductivelayer 130 may be referred to a conductive member or a conductive film.Since the conductive layer 130 electrically connects the oxidesemiconductor 120 and the source and drain electrodes 181 and 182, itmay be referred to as an auxiliary layer or a connecting member.Portions of the conductive layer 130 (i.e. 131, 132) may also bereferred to as conductive members.

The conductive layer 130 is formed on the oxide semiconductor 120. Theconductive layer 130 may include a first conductive layer 131 formed onthe first region 121 of the oxide semiconductor 120 and a secondconductive layer 132 formed on the second region 122 of the oxidesemiconductor 120. The first conductive layer 131 and the secondconductive layer 132 may be formed of the same material but are not indirect contact with each other.

The conductive layer 130 is electrically connected to the oxidesemiconductor 120, and is in contact with the source electrode 181 andthe drain electrode 182 that will be described later, thereby decreasinga resistance between a channel region of the oxide semiconductor 120 andthe source and drain electrodes 181 and 182. Decreasing the resistancebetween the channel region of the oxide semiconductor 120 and the sourceand drain electrodes 181 and 182 by means of the conductive layer 130will be described in more detail later.

The conductive layer 130 may include a conductive material. For example,various conductive metals such as Al, Ti, or the like may be used as theconductive layer 130. The conductive materials of the conductive layer130 are not limited to metals and may be materials having an electronmobility higher than that of the material of the oxide semiconductorlayer 120.

A sputtering process may be employed to form the conductive layer 130 onthe substrate 110 or on the oxide semiconductor 120. A thickness of theconductive layer 130 may be variously determined depending on use andprocess requirements of the thin film transistor 100. When conductivelayer 130 is formed with a large thickness, the conductivity of theconductive layer 130 may be enhanced. In this case, when the conductivelayer 130 is formed with an excessively large thickness, a distancebetween the conductive layer 130 and the gate electrode 160 may bedecreased to cause a leakage current to occur. In addition, when thethickness of the conductive layer 130 is formed to be large, a timetaken for the process to manufacture the conductive layer 130excessively increases. Accordingly, the thickness of the conductivelayer 130 may be properly determined in consideration of theperformance, the design structure, the process requirements, or the likeof the thin film transistor.

The conductive layer 130 may include an oxidizable conductive material.For example, various oxidizable metals such as Al, Ti, or the like maybe formed with proper thicknesses.

A third insulating layer 140 insulating the first conductive layer 131from the second conductive layer 132 may be formed above the thirdregion 123 of the oxide semiconductor 120 and above the first and secondregions 121 and 122 of the oxide semiconductor 120. One side of thethird insulating layer 140 is formed on the first region 121 of theoxide semiconductor 120, and the other side of the third insulatinglayer 140 is formed on the second region 122 of the oxide semiconductor120. The one side and the other side of the third insulating layer 140may be lateral portions opposite to each other.

The third insulating layer 140 is an oxide metal and may be an oxide ofthe oxidizable conductive material constituting the conductive layer130. As described above, the first conductive layer 131 and the secondconductive layer 132 constituting the conductive layer 130 may be formedof the same material, and may be formed in the same process.Accordingly, in order not to have the first conductive layer 131 and thesecond conductive layer 132 in direct contact with each other, anadditional process is required for the region between the firstconductive layer 131 and the second conductive layer 132. In the thinfilm transistor according to an embodiment of the present invention, aportion between the first and second conductive layers 131 and 132, thatis, the oxidizable conductive material corresponding to the third region123 is oxidized to insulate the first conductive layer 131 from thesecond conductive layer 132. A specific process of forming the thirdinsulating layer 140 will be described in more detail later.

As described above, since the third insulating layer 140 is formed byoxidizing the oxidizable conductive material constituting the conductivelayer 130, an oxidization process time of the oxidizable conductivematerial formed on the third region 123 of the oxide semiconductor 120excessively increases when the thickness of the conductive layer 130 isexcessively large. Accordingly, the conductive layer 130 preferably hasa thickness suitable for the oxidization, for example, a thickness in arange from about 30 Å to about 100 Å.

The first insulating layer 150 is formed on the oxide semiconductor 120,the conductive layer 130, and the third insulating layer 140. The firstinsulating layer 150 insulates the oxide semiconductor 120 and theconductive layer 130 from the gate electrode 160. The first insulatinglayer 150 may thus be referred to as a gate insulating film. The firstinsulating layer 150 may be formed of a silicon oxide film, a siliconnitride film, or a stacked layer thereof. However, the first insulatinglayer is not limited thereto and may be formed of various materials.

The first insulating layer 150 may be formed on an entire surface of thesubstrate 110 including the oxide semiconductor 120, the conductivelayer 130, and the third insulating layer 140. However, since the firstinsulating layer 150 only has to insulate the oxide semiconductor 120and the conductive layer 130 from the gate electrode 160, it may beformed on the oxide semiconductor 120 and the conductive layer 130, andin particular, may be formed on third region 123 of the oxidesemiconductor 120. When the first insulating layer 150 is formed on anentire surface of the substrate 110, the first insulating layer 150 maybe formed to have a contact hole exposing some regions of the conductivelayer 130 disposed above the oxide semiconductor 120, and the contacthole may expose some regions of the first conductive layer 131 formed onthe source region of the oxide semiconductor 120 and the secondconductive layer 132 formed on the drain region of the oxidesemiconductor 120. A thickness of the first insulating layer 150 needsto be large enough to insulate the gate electrode 160 from the oxidesemiconductor 120. For example, the first insulating layer may be formedwith a thickness of about 2000 Å. However, the first insulating layer isnot limited thereto.

Since the third insulating layer 140 is formed in the third region 123of the oxide semiconductor 120, it may at least partially overlap thegate electrode 160. The third insulating layer 140 may thus serve as adouble-insulating layer along with the first insulating layer 150. Sincea thickness of the double-insulating layer corresponds to a sum of athickness of the first insulating layer 150 and a thickness of the thirdinsulating layer 140, an insulating property between the oxidesemiconductor 120 and the gate electrode 160 may be enhanced. Asdescribed above, the third insulating layer 140 formed of the metaloxide, along with the first insulating layer 150, may enhance theinsulating property between the oxide semiconductor 120 and the gateelectrode 160, thereby decreasing the occurrence of the leakage current.

The gate electrode 160 is formed on the first insulating layer 150. Thegate electrode 160 is branched from a gate interconnection 161 anddelivers a driving signal delivered via the gate interconnection 161 tothe thin film transistor 100. The gate electrode 160 may at leastpartially overlap the oxide semiconductor 120, and in particular, mayoverlap the third region 123 as the channel region of the oxidesemiconductor 120. The gate electrode 160 may be formed of any one ofMo, Al, Cr, Au, Ti, Ni, Nd, and Cu, or an alloy thereof. However, thegate electrode is not limited thereto and may be formed of variousmaterials. In addition, the gate electrode 160 may be formed of any oneselected from the group consisting of Mo, Al, Cr, Au, Ti, Ni, Nd, andCu, or may be a multi-layer formed of an alloy thereof.

The second insulating layer 170 is formed on the entire surface of thesubstrate 110 including the gate electrode 160. The second insulatinglayer 170 may be referred to as an interlayer insulating film. Thesecond insulating layer 170 may be formed of the same material as thefirst second insulating layer 150, and may be formed of a silicon oxide,a silicon nitride, or a stacked layer thereof. However, the secondinsulating layer 170 is not limited thereto and may be formed of variousmaterials. The second insulating layer 170 may be formed to have acontact hole exposing some regions of the conductive layer 130, and thecontact hole may expose some regions of the first conductive layer 131formed on the source region of the oxide semiconductor 120 and thesecond conductive layer 132 formed on the drain region of the oxidesemiconductor 120.

A contact hole may be formed in at least one of the first insulatinglayer 150 and the second insulating layer 170. The contact hole mayexpose a partial region of the first conductive layer 131 and a partialregion of the second conductive layer 132 so that the source electrode181 and the drain electrode 182 are in contact with the first conductivelayer 131 and the second conductive layer 132, respectively. Asdescribed above, the first insulating layer 150 is a gate insulatinglayer, and may be formed on the third region 123 of the oxidesemiconductor 120 in which the gate electrode 160 is disposed. When thefirst insulating layer 150 is formed on the third region 123 of theoxide semiconductor 120, the contact hole may not be formed in theinsulating layer 150 and may be formed in the second insulating layer170. In addition, when the first insulating layer 150 is formed to coverthe entire surface of the substrate 110, the contact hole may be formedin both of the first and second insulating layers 150 and 170.

The source electrode 181 and the drain electrode 182 are formed on thesecond insulating layer 170. The source electrode 181 is branched from adata interconnection 189 and delivers a data signal delivered via thedata interconnection 189 to the thin film transistor 100. The sourceelectrode 181 and the drain electrode 182 may be in contact with thefirst conductive layer 131 and the second conductive layer 132 formed onthe oxide semiconductor 120 via the contact hole formed in the firstinsulating layer 150 and/or the second insulating layer 170,respectively. As a result, the source electrode 181 may be in contactwith the first conductive layer 131 positioned in the first region 121of the oxide semiconductor 120 to be electrically connected to the firstregion 121 of the oxide semiconductor 120, and the drain electrode 182may be in contact with the second conductive layer 132 positioned in thesecond region 122 of the oxide semiconductor 120 to be electricallyconnected to the second region 122 of the oxide semiconductor 120. Thesource and drain electrodes 181 and 182 may be formed of any one of Mo,Al, Cr, Au, Ti, Ni, Nd, and Cu, or an alloy thereof. However, the sourceand drain electrodes are not limited thereto and may be formed ofvarious materials. In addition, the source and drain electrodes 181 and182 may be formed of any one selected from the group consisting of Mo,Al, Cr, Au, Ti, Ni, Nd, and Cu, or may be a multi-layer formed of analloy thereof.

The source electrode 181 and the drain electrode 182 are in contact withthe first conductive layer 131 and the second conductive layer 132,respectively, thereby decreasing the resistance between the channelregion of the oxide semiconductor 120 and the source and drainelectrodes 181 and 182. When the thin film transistor 100 is driven,electrons move between the source electrode 181 and the channel regionof the oxide semiconductor 120 and between the drain electrode 182 andthe channel region of the oxide semiconductor 120, and thecharacteristics of the thin film transistor 100 is enhanced when theresistance of the region in which the electrons move is smaller. Anelectron movement distance between the source electrode 181 and thechannel region of the oxide semiconductor 120 corresponds to a distancebetween an end portion 183 of the source electrode 181 and the sourceelectrode-side end portion 124 of the channel region of the oxidesemiconductor 120. When the conductive layer 130 is not used, the sourceelectrode 181 is in direct contact with the oxide semiconductor 120, andelectrons must travel a long distance through the oxide semiconductor120 in order to move from the source electrode 181 to the channel regionof the oxide semiconductor 120. However, the conductive layer 130 in thethin film transistor 100 according to an embodiment of the presentinvention, the space between the source electrode 181 and the channelregion of the oxide semiconductor 120 now includes both the firstconductive layer 131 as well as a portion of the gate oxide 120. This isbecause the first conductive layer 131 extends away, from a contact areawhere the source electrode 181 contacts the first conductive layer 131,towards the gate electrode 160, thereby filling part of the spacebetween the source electrode 181 and the channel region 123. As aresult, electrons only need to move through a short portion of the gateoxide 120 that is between the end portion 133 of the first conductivelayer 131 and the source electrode-side end portion 124 of the channelregion of the oxide semiconductor 120. Accordingly, since the distanceby which the electrons move on the oxide semiconductor 120 is shortenedas compared to the case of not using the conductive layer 130, theresistance between the source electrode 181 and the channel region ofthe oxide semiconductor 120 may be relatively decreased, and thecharacteristics of the thin film transistor 100 may also be enhanced. Inaddition, when the conductive layer 130 is used, since the resistancebetween the drain electrode 182 and the channel region of the oxidesemiconductor 120 is relatively decreased for the same reason asdescribed with reference to the source electrode 181, thecharacteristics of the thin film transistor 100 may also be enhanced.

Although it is shown in FIG. 1B that the end portion 133 of the firstconductive layer 131 in contact with the source electrode 181 and towardthe channel region of the oxide semiconductor 120 and the sourceelectrode-side end portion 124 of the channel region of the oxidesemiconductor 120 are spaced from each other, the channel region of theoxide semiconductor 120 and the third insulating layer 140 maycompletely overlap each other so that the end portion 133 of the firstconductive layer 131 in contact with the source electrode 181 and towardthe channel region of the oxide semiconductor 120 may be identical tothe source electrode-side end portion 124 of the channel region of theoxide semiconductor 120. In this case, since the end portion 133 of thefirst conductive layer 131 in contact with the source electrode 181 andtoward the channel region of the oxide semiconductor 120 is in contactwith the channel region of the oxide semiconductor 120 and the space inwhich the electrons move, the space between the source electrode 181 andthe channel region of the oxide semiconductor 120 corresponds to thefirst conductive layer 131 that is completely conductive, the resistancebetween source electrode 181 and the channel region of the oxidesemiconductor 120 may be relatively decreased and the characteristics ofthe thin film transistor 100 may also be enhanced. In addition, it isshown in FIG. 1B that an end portion 134 of the second conductive layer132 in contact with the drain electrode 182 and toward the channelregion of the oxide semiconductor 120 and the drain electrode-side endportion 125 of the channel region of the oxide semiconductor 120 arespaced from each other. However, the end portion 134 of the secondconductive layer 132 in contact with the drain electrode 182 toward thechannel region of the oxide semiconductor 120 may be in contact with thechannel region of the oxide semiconductor 120, and the resistancebetween the drain electrode 182 and the channel region of the oxidesemiconductor 120 may thus be relatively decreased and thecharacteristics of the thin film transistor 100 may also be enhanced.

A distance d₁ from the end portion 133 of the first conductive layer 131in contact with the source electrode 181 and toward the channel regionof the oxide semiconductor 120 to the source electrode-side end portion124 of the channel region of the oxide semiconductor 120 may be equal toor less than a contact distance. In addition, a distance d₂ from the endportion 134 of the second conductive layer 132 in contact with the drainelectrode 182 and toward the channel region of the oxide semiconductor120 to the drain electrode-side end portion 125 of the channel region ofthe oxide semiconductor 120 may also be equal to or less than thecontact distance. The contact distance as used herein is a distance fornormal driving of the thin film transistor 100, and indicates a shortestdistance in the oxide semiconductor 120 in which electrons can movebetween the conductive layer 130 and the channel region of the oxidesemiconductor 120. For example, the contact distance may be defined tobe about 500 Å or less.

The thin film transistor may be classified as an inverted-staggeredstructure or a coplanar structure depending on positions of the activelayer, the gate electrode, the source electrode, and the drainelectrode. The thin film transistor of the coplanar structure is a thinfilm transistor having a structure in which not only the source anddrain electrodes but also the gate electrode are positioned above theactive layer and all of the source electrode, the drain electrode, andthe gate electrode are thus positioned on the same side with the activelayer being a reference therebetween, and may be referred to as a topgate thin film transistor. The thin film transistor of theinverted-staggered structure is a thin film transistor having astructure in which the source and drain electrodes are positioned abovethe active layer and the gate electrode is positioned below the activelayer and the source and drain electrodes are positioned on the oppositeside as the gate electrode with the active layer being a referencetherebetween, and may be referred to as a bottom gate thin filmtransistor. In the case of the thin film transistor having theinverted-staggered structure, a parasitic capacitance between the gateelectrode and the source and drain electrodes is significantly high andit is thus difficult to apply the thin film transistor having theinverted-staggered structure to a high-resolution display. However, inthe case of the thin film transistor having the coplanar structure suchas the thin film transistor according to an embodiment of the presentinvention, the parasitic capacitance between the gate electrode and thesource and drain electrodes is relatively low, and it is thus possibleto readily apply the thin film transistor having the coplanar structureto the high-resolution display. It is assumed herein that the thin filmtransistor is the thin film transistor having the coplanar structure.

FIG. 2A is a plan view illustrating a thin film transistor in accordancewith another embodiment of the present invention. FIG. 2B is across-sectional view illustrating a thin film transistor taken along theline IIb-IIb′ of FIG. 2A. Since a substrate 210, a first insulatinglayer 250, a gate electrode 260, a gate interconnection 261, a secondinsulating layer 270, a source electrode 281, a drain electrode 282, anda data interconnection 289 are substantially the same as the substrate110, the first insulating layer 150, the gate electrode 160, the gateinterconnection 161, the second insulating layer 170, the sourceelectrode 181, the drain electrode 182, and the data interconnection 189of FIGS. 1A and 1B, an overlapping description thereof will be omitted.

The conductive layer 230 is formed above the oxide semiconductor 220.The conductive layer 230 may include a first conductive layer 231 formedon the first region 221 of the oxide semiconductor 220 and a secondconductive layer 232 formed on the second region 222 of the oxidesemiconductor 220. The first conductive layer 231 and the secondconductive layer 232 may be formed of the same material but iselectrically isolated from each other.

The conductive layer 230 is electrically connected to the oxidesemiconductor 220, and is in contact with the source electrode 281 andthe drain electrode 282 to decrease a resistance between the oxidesemiconductor 220 and the source and drain electrodes 281 and 282.

The conductive layer 230 may include a transparent conductive material,and may include a material having a substantially similar etchingcharacteristic to that of the oxide semiconductor 220. The transparentconductive material that may be included in the conductive layer 230 mayinclude, for example, a transparent conductive oxide (TCO),particularly, an indium tin oxide (ITO), an indium zinc oxide (IZO), orthe like.

The first conductive layer 231 and the second conductive layer 232 maybe formed by etching the conductive layer 230. When the conductive layer230 formed on the third region 223 of the oxide semiconductor 220 isetched after the transparent conductive material is formed on an entiresurface of the oxide semiconductor 220, the first conductive layer 231and the second conductive layer 232 of the conductive layer 230 may notbe in direct contact with each other. A specific process of forming thefirst conductive layer 231 and the second conductive layer 232 will bedescribed later.

A height h₃ of the oxide semiconductor 220 in the third region 223 maybe equal to or less than heights h₁ and h₂ of the oxide semiconductor220 in the first and second regions 221 and 222. As described above, theconductive layer 230 formed on the third region 223 of the oxidesemiconductor 220 may be etched in order to electrically isolate thefirst conductive layer 231 from the second conductive layer 232.However, since etching characteristics of the materials constituting theoxide semiconductor 220 and the conductive layer 230 are substantiallysimilar to each other, accurately removing only the conductive layer 230up to the boundary surface between the conductive layer 230 and theoxide semiconductor 220 is a significantly difficult process. The heighth₃ of the oxide semiconductor 220 in the third region 223 may thus beequal to or less than the heights h₁ and h₂ of the oxide semiconductor220 in the first and second regions 221 and 222.

The surface characteristic of the third region 223 of the oxidesemiconductor 220 may be different from those of the first region 221and the second region 222 of the oxide semiconductor 220. The firstregion 221 and the second region 222 of the oxide semiconductor 220correspond to regions in which the conductive layer 230 is formed afterthe oxide semiconductor 220 is formed. Accordingly, surfaces of thefirst region 221 and the second region 222 of the oxide semiconductor220, that is, upper surfaces of the first region 221 and the secondregion 222 of the oxide semiconductor 220 are surfaces to which theetching process is not applied. The third region 223 of the oxidesemiconductor 220 is a region in which the conductive layer 230 isremoved after the oxide semiconductor 220 is formed and the conductivelayer 230 is formed. Accordingly, the surface of the third region 223 ofthe oxide semiconductor 220, that is, an upper surface of the thirdregion 223 of the oxide semiconductor 220 is a surface to which theetching process is applied. The surface characteristics of the thirdregion 223 of the oxide semiconductor 220 may thus be different fromthose of the first region 221 and the second region 222 of the oxidesemiconductor 220 in terms of a roughness, a molecular or atomic bondingforce, a molecular composition ratio, or the like.

The source electrode 281 and the drain electrode 282 are in contact withthe first conductive layer 231 and the second conductive layer 232,respectively, thereby decreasing the resistance between the channelregion of the oxide semiconductor 220 and the source and drainelectrodes 281 and 282. When the thin film transistor 200 is driven,electrons between the source electrode 281 and the channel region of theoxide semiconductor 220 and between the drain electrode 282 the channelregion of the oxide semiconductor 220 move, and the characteristics ofthe thin film transistor 200 is enhanced when the resistance of theregion in which the electrons move is smaller. A distance between thesource electrode 281 and the channel region of the oxide semiconductor220, that is, an electron movement distance corresponds to a distancebetween the end portion 283 of the source electrode 281 and the sourceelectrode-side end portion 224 of the channel region of the oxidesemiconductor 220. When the conductive layer 230 is not used, the sourceelectrode 281 is in direct contact with the oxide semiconductor 220, andelectrons must travel a long distance through the oxide semiconductor220 in order to move from the source electrode 281 to the channel regionof the oxide semiconductor 220. However, when the conductive layer 230is used as in the thin film transistor 200 according to an embodiment ofthe present invention, the space between the source electrode 281 andthe channel region of the oxide semiconductor 220 now includes both thefirst conductive layer 231 as well as a portion of the gate oxide 220.As a result, electrons only need to move through a short portion of thegate oxide 220 that is between the end portion 233 of the firstconductive layer 231 and the source electrode-side end portion 224 ofthe channel region of the oxide semiconductor 220. Accordingly, sincethe distance by which the electrons move on the oxide semiconductor 220is shortened as compared to the case of not using the conductive layer230, the resistance between the source electrode 281 and the channelregion of the oxide semiconductor 220 may be relatively decreased, andthe characteristics of the thin film transistor 200 may also beenhanced. In addition, when the conductive layer 230 is used, since theresistance between the drain electrode 282 and the channel region of theoxide semiconductor 220 is relatively decreased for the same reason asdescribed with reference to the source electrode 281, thecharacteristics of the thin film transistor 200 may also be enhanced.

A distance d₁ from the end portion 233 of the first conductive layer 231in contact with the source electrode 281 and toward the channel regionof the oxide semiconductor 220 to the source electrode-side end portion224 of the channel region of the oxide semiconductor 220 may be equal toor less than the contact distance. In addition, a distance d₂ from theend portion 234 of the second conductive layer 232 in contact with thedrain electrode 282 and toward the channel region of the oxidesemiconductor 220 to the drain electrode-side end portion 225 of thechannel region of the oxide semiconductor 220 may also be equal to orless than the contact distance. The distance d₁ may be zero such thatthe end portion 233 of the first conductive layer 231 is aligned withthe end 224 of the channel region. The distance d₂ may similarly be zerosuch that the end portion 234 of the second conductive layer 232 isaligned with the end 225 of the channel region.

FIG. 3A is a plan view illustrating a thin film transistor in accordancewith yet another embodiment of the present invention. FIG. 3B is across-sectional view illustrating a thin film transistor taken along theline IIIb-IIIb′ of FIG. 3A. Since a substrate 310, an oxidesemiconductor 320, a first insulating layer 350, a gate electrode 360, asecond insulating layer 370, a source electrode 381, a drain electrode382, a gate interconnection 361, and a data interconnection 389 aresubstantially the same as the substrate 110, the oxide semiconductor120, the conductive layer 130, the first insulating layer 150, the gateelectrode 160, the second insulating layer 170, the source electrode181, the drain electrode 182, the gate interconnection 161, and the datainterconnection 189 of FIGS. 1A and 1B, an overlapping descriptionthereof will be omitted.

A conductive layer 330 is formed at one side of the oxide semiconductor320 to be electrically connected to the oxide semiconductor 320. Theconductive layer 330 may be referred to as a conductive member or aconductive film. Since the conductive layer 330 electrically connectsthe oxide semiconductor 320 to the source electrode 381 and the drainelectrode 382 that will be described later, it may also be referred toas an auxiliary layer or a connecting member.

The conductive layer 330 is formed at a side surface of the oxidesemiconductor 320. The conductive layer 330 may include a firstconductive layer 331 formed at one side of the oxide semiconductor 320and a second conductive layer 332 formed on the other side of the oxidesemiconductor 320. The first conductive layer 331 and the secondconductive layer 332 may be formed of the same material on the sameplane but are not in direct contact with each other. Although the oneside and the other side of the oxide semiconductor 320 are shown in arectangular shape in FIG. 3B, the one side and the other side may belaterally inclined in other embodiments.

The conductive layer 330 may include a conductive material, and mayinclude, for example, not only various oxidizable metals such as Al orTi but also transparent conductive materials such as ITO or IZO.

The height of the conductive layer 330 may be substantially equal to orless than the height of the oxide semiconductor 320. The height of theconductive layer 330 is not particularly limited. However, when theheight of the conductive layer 330 is greater than the height of theoxide semiconductor 320, a distance between the conductive layer 330 andthe gate electrode 360 is shortened, thereby increasing the leakagecurrent. Accordingly, the height of the conductive layer 330 may beformed to be substantially equal to or less than the height of the oxidesemiconductor 320.

An area of the conductive layer 330 may be determined to meet the areadesign of the thin film transistor 300. The area of the conductive layer330 is not particularly limited. However, since the conductive layer 330is formed on the same plane as the oxide semiconductor 320, it may bedifficult to implement the high-resolution display when the area of theconductive layer 330 increases. Accordingly, the area of the conductivelayer 330 may be determined based on the area design of the thin filmtransistor 300.

The oxide semiconductor 320 used as the active layer and the source anddrain electrodes 381 and 382 are electrically connected to each othervia the conductive layer 330. Since the oxide semiconductor 320 has asemiconductive property and the source and drain electrodes 381 and 382have conductive properties, a resistance occurs between the source anddrain electrodes 381 and 382 and the channel region of the oxidesemiconductor 320, that is, the region of the oxide semiconductor 320overlapping the gate electrode 360, and the electric characteristic ofthe thin film transistor 300 may be deteriorated. In the thin filmtransistor 300 according to another embodiment of the present invention,the conductive layer 330 may be formed of the material allowing theresistance between the oxide semiconductor 320 and the source and drainelectrodes 381 and 382 to be minimized between the channel region of theoxide semiconductor 320 and the source and drain electrodes 381 and 382,thereby enhancing the characteristics of the thin film transistor 300.

In the present embodiment, the conductive layer 330 may be positionedfrom the channel region of the oxide semiconductor 320 with apredetermined interval therebetween. A distance d₁ from the end portion333 of the first conductive layer 331 in contact with the sourceelectrode 381 and toward the channel region of the oxide semiconductor320 to the source electrode-side end portion 324 of the channel regionof the oxide semiconductor 320 may be equal to or less than the contactdistance. In addition, a distance d₂ from the end portion 334 of thesecond conductive layer 332 in contact with the drain electrode 382 andtoward the channel region of the oxide semiconductor 320 to the drainelectrode-side end portion 325 of the channel region of the oxidesemiconductor 320 may also be equal to or less than the contactdistance.

FIG. 4A is a cross-sectional view illustrating a thin film transistor inaccordance with another embodiment of the present invention. Since asubstrate 410A, a first insulating layer 450A, a gate electrode 460A, asecond insulating layer 470A, a source electrode 481A, and a drainelectrode 482A are substantially the same as the substrate 110, thefirst insulating layer 150, the gate electrode 160, the secondinsulating layer 170, the source electrode 181, and the drain electrode182 of FIGS. 1A and 1B, an overlapping description thereof will beomitted.

An active structure 490A is formed on the substrate 410A. The activestructure 490A indicates a structure serving as the active layer bymeans of combination of one or more elements. The active structure 490Aincludes an oxide semiconductor 420A, and a conductive layer 430Aelectrically connecting the oxide semiconductor 420A and the source anddrain electrodes 481A and 482A.

The oxide semiconductor 420A of the active structure 490A is formed asan active layer on the substrate 410A, and the conductive layer 430A isformed on the oxide semiconductor 420A to decrease the resistancebetween the oxide semiconductor 420A and the source and drain electrodes481A and 482A. The conductive layer 430A may include a first conductivelayer 431A formed on the first region 421A of the oxide semiconductor420A and a second conductive layer 432A formed on the second region 422Aof the oxide semiconductor 420A. The first conductive layer 431A may beformed to be in contact with and between the source electrode 481A andthe oxide semiconductor 420A to exhibit a conductivity between thesource electrode 481A and the oxide semiconductor 420A, and the secondconductive layer 432A may be formed to be in contact with and betweenthe drain electrode 482A and the oxide semiconductor 420A to exhibit aconductivity between the drain electrode 482A and the oxidesemiconductor 420A. The first conductive layer 431A and the secondconductive layer 432A may be formed of various oxidizable metals. Sincethe oxide semiconductor 420A and the conductive layer 430A aresubstantially the same as the oxide semiconductor 120 and the conductivelayer 130 of FIGS. 1A and 1B, an overlapping description thereof will beomitted.

FIG. 4B is a cross-sectional view illustrating a thin film transistor inaccordance with another embodiment of the present invention. Since asubstrate 410B, a first insulating layer 450B, a gate electrode 460B, asecond insulating layer 470B, a source electrode 481B, and a drainelectrode 482B are substantially the same as the substrate 210, thefirst insulating layer 250, the gate electrode 260, the secondinsulating layer 270, the source electrode 281, and the drain electrode282 of FIGS. 2A and 2B, an overlapping description thereof will beomitted.

An active structure 490B is formed on the substrate 410B. The activestructure 490B indicates a structure serving as the active layer bymeans of combination of one or more elements. The active structure 490Bincludes an oxide semiconductor 420B, and a conductive layer 430Belectrically connecting the oxide semiconductor 420B and the source anddrain electrodes 481B and 482B.

The oxide semiconductor 420B of the active structure 490B is formed asan active layer on the substrate 410B, and the conductive layer 430B isformed on the oxide semiconductor 420B to decrease the resistancebetween the oxide semiconductor 420B and the source and drain electrodes481B and 482B. The conductive layer 430B may include a first conductivelayer 431B formed on the first region 421B of the oxide semiconductor420B and a second conductive layer 432B formed on the second region 422Bof the oxide semiconductor 420B. The first conductive layer 431B may beformed to be in contact with and between the source electrode 481B andthe oxide semiconductor 420B to exhibit a conductivity between thesource electrode 481B and the oxide semiconductor 420B, and the secondconductive layer 432B may be formed to be in contact with and betweenthe drain electrode 482B and the oxide semiconductor 420B to exhibit aconductivity between the drain electrode 482B and the oxidesemiconductor 420B. The first conductive layer 431B and the secondconductive layer 432B may be formed of transparent conductive materials.Since the oxide semiconductor 420B and the conductive layer 430B aresubstantially the same as the oxide semiconductor 220 and the conductivelayer 230 of FIGS. 2A and 2B, an overlapping description thereof will beomitted.

FIG. 4C is a cross-sectional view illustrating a thin film transistor inaccordance with another embodiment of the present invention. Since asubstrate 410C, a first insulating layer 450C, a gate electrode 460C, asecond insulating layer 470C, a source electrode 481C, and a drainelectrode 482C are substantially the same as the substrate 310, thefirst insulating layer 350, the gate electrode 360, the secondinsulating layer 370, the source electrode 381, and the drain electrode382 of FIGS. 3A and 3B, an overlapping description thereof will beomitted.

An active structure 490C is formed on the substrate 410C. The activestructure 490C indicates a structure serving as the active layer bymeans of combination of one or more elements. The active structure 490Cincludes an oxide semiconductor 420C, and a conductive layer 430Celectrically connecting the oxide semiconductor 420C and the source anddrain electrodes 481C and 482C.

The oxide semiconductor 420C of the active structure 490C is formed asan active layer on the substrate 410C, and the conductive layer 430C isformed at one side of the oxide semiconductor 420C to decrease theresistance between the oxide semiconductor 420C and the source and drainelectrodes 481C and 482C. The conductive layer 430C may include a firstconductive layer 431C formed at one side of the first region 421C of theoxide semiconductor 420C and a second conductive layer 432C formed atone side of the second region 422C of the oxide semiconductor 420C. Thefirst conductive layer 431C may be formed to be in contact with andbetween the source electrode 481C and the oxide semiconductor 420C toexhibit a conductivity between the source electrode 481C and the oxidesemiconductor 420C, and the second conductive layer 432C may be formedto be in contact with and between the drain electrode 482C and the oxidesemiconductor 420C to exhibit a conductivity between the drain electrode482C and the oxide semiconductor 420C. The first conductive layer 431Cand the second conductive layer 432C are conductive materials, and maybe formed of various oxidizable metals or transparent conductivematerials. Since the oxide semiconductor 420C and the conductive layer430C are substantially the same as the oxide semiconductor 320 and theconductive layer 330 of FIGS. 3A and 3B, an overlapping descriptionthereof will be omitted.

FIG. 5 is a schematic diagram illustrating a display device to whichthin film transistors can be applied in accordance with variousembodiments of the present invention. The display device is a device fordisplaying images using transistors that control the operation ofdisplay elements, and includes various display devices such as anorganic light emitting diode (OLED) display, a liquid crystal display(LCD), an electrophoretic display (EPD), or the like.

The display device 500 may be an OLED display, and the display device500 includes a substrate, a plurality of thin film transistors, and anorganic light-emitting element having an anode, an organiclight-emitting layer and a cathode. Thin film transistors for allowingthe organic light-emitting layer to emit light are included in aplurality of pixel regions SP of the display panel 510 of the displaydevice 500. For example, as shown in FIG. 5, when a scan signal isapplied from the gate driver 520 to the thin film transistors, the thinfilm transistors may include a switching thin film transistor deliveringa data signal from the data driver 530 to a gate electrode of a drivingthin film transistor, and the driving thin film transistor deliveringthe current delivered through the power supply 540 by the data signaldelivered from the switching thin film transistor to the anode andcontrolling emission of the organic light-emitting layer of thecorresponding pixel or subpixel by means of the current delivered to theanode. Although not shown in FIG. 5, a thin film transistor for acompensation circuit preventing abnormal driving of the display devicemay be included. The thin film transistors of the display device 500 maybe ones of thin film transistors according to various embodiments of thepresent invention.

The display device may be an LCD, and the display device includes alower substrate, an upper substrate, pixel electrodes, a commonelectrode, a color filter, and a liquid crystal layer interposed betweenthe upper and lower substrate. The display device includes a pluralityof pixel regions, and a plurality of thin film transistors forindependently driving the plurality of pixel regions. The thin filmtransistors are electrically connected to the pixel electrodes formed inthe lower substrate of the respective pixel regions to apply a voltagefor each pixel electrode, and orient the liquid crystal by means of theelectric field formed between the common electrode formed in the upperor lower substrate and the pixel electrodes formed in the respectivepixel regions. The oriented liquid crystal allows light emitted from aseparate light source to be selectively transmitted. The selectivelytransmitted light passes through the color filter disposed in the uppersubstrate to display an image. The thin film transistors of the displaydevice may be ones of thin film transistors according to variousembodiments of the present invention.

The display device may be an EPD, and the display device includes alower substrate, an upper substrate, pixel electrodes, a commonelectrode, an optical medium layer. The optical medium layer isinterposed between the upper and lower substrates, and includes a fluidand colored and charged particles dispersed in the fluid. The displaydevice includes a plurality of pixel regions, and a plurality of thinfilm transistors for independently driving the pixel regions. The thinfilm transistors are electrically connected to the pixel electrodesformed in the lower substrate of the respective pixel regions to apply avoltage for each pixel electrode, and move the colored and chargedparticles by means of the electric field formed between the commonelectrode formed in the upper substrate and the pixel electrodesdisposed in the respective pixel regions. The display device moves thecolored and charged particles as described above, and colors of thecolored and charged particles are displayed when the colored and chargedparticles are positioned in the front surface of the display device, forexample, in the upper substrate. The thin film transistors of thedisplay device may be ones of the thin film transistors according tovarious embodiments of the present invention.

The thin film transistors according to various embodiments of thepresent invention may be used in various applications. For example, thethin film transistors may be used in various display devices, and thedisplay device 500 may be applied to not only the OLED but also the LCD,the EPD, or the like.

When the thin film transistors according to various embodiments of thepresent invention are used in the display device, the design of the thinfilm transistor may be partially changed by the kind of the displaydevice. For example, in the case of the flexible display device, sincethe display device needs to be repeatedly bent or folded, variouselements constituting the thin film transistor facilitating bending orfolding may also be employed. In addition, in the case of thetransparent display device, when the display is seen from one surface,the opposite surface of the display device needs to be visible to someextent, and sizes of various elements constituting the thin filmtransistor may thus be designed to be very small or the various elementsconstituting the thin film transistor may be formed of a transparentmaterial.

When the thin film transistors according to various embodiments of thepresent invention are used in the display device, the design of the thinfilm transistors may be partially changed by the installation items ofthe display device. For example, when the display device is installed insmall-sized devices or mobile devices such as a cellular phone, a smartphone, a tablet PC, or a PDA, since these devices have batteries withoutexternal power sources, elements of the thin film transistor may bedesigned to be suitable for the limited battery capacity. In addition,when the display device is installed in large-sized devices or fixeddevices such as a television, a monitor, a screen, or an electronicdisplay board, the external power source is supplied, and elements ofthe thin film transistor may thus be designed to allow the displaydevice to implement the higher resolution under the stable supplycondition of the power source.

When the thin film transistors according to various embodiments of thepresent invention are used in the display device, the design of the thinfilm transistors may be partially changed by the installation place ofthe display device. For example, when the display device is installed inplaces having a large amount of moisture such as a rest room, a basin, ashower stall, a kitchen, the thin film transistors may be designed to beformed of elements having a high resistance to moisture. In addition,when the display device is installed in positions that are apt to beexposed to external impacts such as an outer wall of a building, abuilding glass, a vehicle glass, or the like, the thin film transistorsmay be designed to be formed of elements having a high resistance to theimpact or easily absorbing the impact.

The thin film transistors according to various embodiments of thepresent invention are not limited to the modified examples describedabove and may be applied to various applications, and the design of thethin film transistors may be variously changed in accordance withapplications to which the thin film transistors are applied.

FIG. 6 is a flowchart illustrating a method of manufacturing a thin filmtransistor in accordance with an embodiment of the present invention.FIGS. 7A to 7F are cross-sectional views of respective processesillustrating a method of manufacturing a thin film transistor inaccordance with an embodiment of the present invention.

First, an oxide semiconductor is formed on a substrate (S600), and aconductive layer is formed to be electrically connected to the oxidesemiconductor (S601). A process of forming the oxide semiconductor andthe conductive layer will be described in more detail with reference toFIGS. 7A and 7B.

Referring to FIG. 7A, an oxide semiconductor 720 and an intermediateconductive layer 736 may be formed on a substrate 710. Forming the oxidesemiconductor 720 and the intermediate conductive layer 736 on thesubstrate 710 may include depositing a material for the oxidesemiconductor and the intermediate conductive layer 736 on an entiresurface of the substrate 710 and then selectively patterning thematerial for the oxide semiconductor 720 and the intermediate conductivelayer 736 by means of a photolithography process.

Referring to FIG. 7B, a conductive layer 730 may be formed on the oxidesemiconductor 720. Forming the conductive layer 730 may includeselectively patterning the only intermediate conductive layer 736 on thethird region 723 of the oxide semiconductor 720 on the oxidesemiconductor 720 in which the intermediate conductive layer 736 isformed using the photolithography process. The intermediate conductivelayer 736 on the third region 723 of the oxide semiconductor 720 may bepatterned by the back-etching process, and the conductive layer 730 maythus be divided into a first conductive layer 731 and a secondconductive layer 732 formed of the same material. However, the firstconductive layer 731 and the second conductive layer 732 are not indirect contact with each other.

A first insulating layer is then formed on the substrate (S602), and agate electrode is formed on the first insulating layer (S603). A processof forming the first insulating layer and the gate electrode will bedescribed in more detail with reference to FIG. 7C.

Referring to FIG. 7C, forming the first insulating layer 750 and thegate electrode 760 may include forming the first insulating layer 750with a silicon oxide, a silicon nitride, or a stacked layer thereof onan entire surface of the substrate 710 in which the oxide semiconductor720 and the conductive layer 730 are formed or on the third region 723of the oxide semiconductor 720, depositing a material for the gateelectrode on the entire surface of the first insulating layer 750, andthen selectively patterning the material for the gate electrode by meansof the photolithography process.

A second insulating layer is then formed on the substrate (S604), and afirst contact hole and a second contact hole are formed in at least oneof the first and second insulating layers (S605). A processing offorming the second insulating layer and the first and second contactholes will be described in more detail with reference to FIG. 7D.

Referring to FIG. 7D, forming the second insulating layer 770 mayinclude forming the second insulating layer 770 with a silicon oxide, asilicon nitride, or a stacked layer thereof on the entire surface of thesubstrate 710 in which the first insulating layer 750 and the gateelectrode 760 are formed or on the gate electrode 760. Forming the firstand second contact holes includes forming the first and second contactholes in at least one of the first insulating layer 750 and the secondinsulating layer 770. The first and second contact holes may be formedin both of the first insulating layer 750 and the second insulatinglayer 770 when the first insulating layer 750 and the second insulatinglayer 770 are formed on the entire surface of the substrate 710.However, the first and second contact holes may be formed in only one ofthe first insulating layer 750 and the second insulating layer 770 whenany one of the first insulating layer 750 and the second insulatinglayer 770 is formed only in a partial region of the substrate 710, forexample, only in a region overlapping the gate electrode 760.

Next, each of source and drain electrodes is formed to be electricallyconnected to the conductive layer via the first and second contact holes(S606). A process of forming the source and drain electrodes will bedescribed in more detail with reference to FIG. 7E.

Referring to FIG. 7E, forming the source electrode 781 and the drainelectrode 782 may include filling the first and second contact holeswith a material for the source and drain electrodes while forming thematerial for the source and drain electrodes on the entire surface ofthe substrate 710, and selectively patterning the material for thesource and drain electrodes by means of the photolithography process.

In addition, referring to FIG. 7F, a fourth insulating layer 775 may beformed on the entire surface of the substrate 710 including the sourceelectrode 781 and the drain electrode 782. The fourth insulating layer775 is formed on the entire surface of the substrate 710 including thesource electrode 781 and the drain electrode 782, and may thus act toprotect elements disposed below the fourth insulating layer 775. Thefourth insulating layer 775 may be referred to as a passivation film,and may be formed of a silicon oxide, a silicon nitride, or a stackedlayer thereof. However, the fourth insulating layer is not limitedthereto and may be formed of various materials.

In addition, an overcoating layer may be formed on the entire surface ofthe substrate 710 including the source electrode 781 and the drainelectrode 782. The overcoating layer is formed on the entire surface ofthe substrate 710 including the source electrode 781 and the drainelectrode 782, and may thus protect elements disposed below the fourthinsulating layer 775 and may also planarize the substrate 710 to readilyallow other elements to be formed or disposed above the thin filmtransistor 700. The overcoating layer may be formed of at least onematerial of a polyacrylate resin, an epoxy resin, a phenolic resin, apolyamide resin, a polyimide rein, an unsaturated polyester resin, apoly-phenylenether resin, a poly-phenylenesulfide resin, andbenzocyclobutene.

FIGS. 8A to 8G are cross-sectional views of respective processesillustrating a method of manufacturing a thin film transistor inaccordance with another embodiment of the present invention.

First, an oxide semiconductor is formed on a substrate (S600), and aconductive layer is formed to be electrically connected to the oxidesemiconductor (S601). A process of forming the oxide semiconductor andthe conductive layer will be described in more detail with reference toFIGS. 8A to 8G.

Referring to FIG. 8A, an intermediate oxide semiconductor layer 826 andan intermediate conductive layer 836 may be deposited on an entiresurface of the substrate 810, and then a photoresist 890 as aphotosensitive material may be formed on the intermediate oxidesemiconductor layer 826 and the intermediate conductive layer 836. It isassumed herein that the photoresist 890 is a positive type photoresistfor convenience of description.

Next, referring to FIGS. 8B and 8C, a diffraction exposure mask 895 maybe used to selectively expose the photoresist 890. The diffractionexposure mask 895 is a mask also referred to as a halftone mask, andindicates a mask having a different light transmittance for each region.The diffraction exposure mask 895 includes a first transmission regionin which all radiated light is transmitted, a second transmission region897 in which only some of the radiated light is transmitted, and alight-blocking region 898 in which all radiated light is blocked. Sincethe photoresist 890 is the positive type photoresist 890, after thediffracted and exposed photoresist 890 is developed, the photoresist 890corresponding to the first transmission region 896 is fully removed, thephotoresist 890 corresponding to the second transmission region 897 ispartially removed, and the photoresist 890 corresponding to thelight-blocking region 898 is not substantially removed as shown in FIG.8C. Accordingly, first and second photoresist layers having a firstheight are formed in the regions corresponding to the first region 821and the second region 822 of the oxide semiconductor 820, a thirdphotoresist layer having a second height is formed in the regioncorresponding to the third region 823 of the oxide semiconductor 820,and the first height is greater than the second height. It is assumedthat the positive type photoresist is used for convenience ofdescription. However, a negative type photoresist may be employed. Whenthe negative type photoresist is used, the exposed and developed resultsin the first transmission region and the light-blocking region are viceversa.

In some embodiments, a plurality of masks other than the singlediffraction exposure mask 895 as described above may be used in order toform the photoresist 891. For example, a first mask having the sametransmission region as the region corresponding to the firsttransmission region 896 of the diffraction exposure mask 895 may be usedto remove the photoresist 890 in the region corresponding to the firsttransmission region 896 of the diffraction exposure mask 895 and asecond mask having the same transmission region as the regioncorresponding to the second transmission region 897 of the diffractionexposure mask 895 may be used to partially remove the photoresist 890 inthe region corresponding to the second transmission region 897 of thediffraction exposure mask 895 in the condition in which the photoresist890 is formed as shown in FIG. 8B, thereby forming the photoresist 891.Alternatively, the second mask may be used first and then the first maskmay be used to form the photoresist 891. However, the present inventionis not limited thereto and a plurality of masks having varioustransmission regions may be used to form the photoresist 891.

Next, referring to FIG. 8D, the photoresist 891 formed as shown in FIG.8D may be used as a mask to selectively remove the intermediate oxidesemiconductor layer 826 and the intermediate conductive layer 836,thereby forming the oxide semiconductor 820.

An ashing process may then be performed on the remaining photoresist 891to decrease the height of the photoresist 892 by generally the samedegree as shown in FIG. 8E, and the ashing process may be performeduntil the portion of the intermediate conductive layer 836 correspondingto the third region 823 (as shown in FIG. 8G) of the oxide semiconductor820 is exposed.

Next, referring to FIG. 8F, the portion of the intermediate conductivelayer 836 exposed by the ashing process and corresponding to the thirdregion 823 of the oxide semiconductor 820 may be oxidized. Accordingly,a conductive layer 830 including a first conductive layer 831 and asecond conductive layer 832, and a third insulating layer 840 betweenthe first conductive layer 831 and the second conductive layer 832 maybe formed. After the oxidization process is completed, the remainingphotoresist 892 may be removed.

Next, referring to FIG. 8G, a first insulating layer 850 is formed on asubstrate 810 (S602), a gate electrode 860 is formed on the firstinsulating layer 850 (S603), a second insulating layer 870 is formed onthe substrate 810 (S604), first and second contact holes are formed inat least one of the first and second insulating layers 850 and 870(S605), and each of source and drain electrodes 881 and 882 is formed tobe electrically connected to the conductive layer 830 via the first andsecond contact holes (S606). Since steps S602 to S606 are substantiallythe same as steps S602 to S606 described with reference to FIGS. 7A to7F, an overlapping description thereof will be omitted.

FIGS. 9A to 9D are cross-sectional views of respective processesillustrating a method of manufacturing a thin film transistor inaccordance with yet another embodiment of the present invention.

First, an oxide semiconductor is formed on a substrate (S600). A processof forming the oxide semiconductor will be described in more detail withreference to FIG. 9A.

Referring to FIG. 9A, forming the oxide semiconductor 920 on thesubstrate 910 may include depositing a material for the oxidesemiconductor on an entire surface of the substrate 910, and thenselectively patterning the material for the oxide semiconductor by meansof the photolithography process.

A conductive layer is then formed to be electrically connected to theoxide semiconductor (S601). A process of forming the conductive layerwill be described in more detail with reference to FIGS. 9B and 9C.

Referring to FIG. 9B, forming the conductive layer 930 may includeforming an intermediate second conductive layer 936 on a region of thesubstrate 910 in which the oxide semiconductor 920 is not formed, andforming a photoresist 990 on a region in which the oxide semiconductor920 and the conductive layer 930 are to be formed.

Next, referring to FIG. 9C, forming the conductive layer 930 may includeselectively patterning the intermediate second conductive layer 936using the photoresist 990 as a mask to form the conductive layer 930.

Next, referring to FIG. 9D, a first insulating layer 950 is formed on asubstrate 910 (S602), a gate electrode 960 is formed on the firstinsulating layer 950 (S603), a second insulating layer 970 is formed onthe substrate 910 (S604), first and second contact holes are formed inat least one of the first and second insulating layers 950 and 970(S605), and source and drain electrodes are formed to be electricallyconnected to the conductive layer 930 via the first and second contactholes, respectively (S606). Since steps S602 to S606 are substantiallythe same as steps S602 to S606 described with reference to FIGS. 7A to7F, an overlapping description thereof will be omitted.

FIG. 10 is a flowchart illustrating a method of manufacturing a thinfilm transistor in accordance with another embodiment of the presentinvention.

First, an active structure including an oxide semiconductor is formed ona substrate (S1000). The active structure indicates a structure servingas the active layer by means of combination of one or more elements. Theactive structure includes an oxide semiconductor, and a conductive layerelectrically connecting the oxide semiconductor and source and drainelectrodes. Since forming the oxide semiconductor and the source anddrain electrodes included in the active structure is substantially thesame as forming the oxide semiconductor and the conductive layer ofFIGS. 7A to 9D, an overlapping description thereof will be omitted.

Next, a first insulating layer is formed on a substrate (S1001), a gateelectrode is formed on the first insulating layer (S1002), a secondinsulating layer is formed on the substrate (S1003), first and secondcontact holes are formed in at least one of the first and secondinsulating layers (S1004), and source and drain electrodes are formed tobe electrically connected to the active structure via the first andsecond contact holes, respectively (S1005). Since steps S1001 to S1005are substantially the same as steps S602 to S606 described withreference to FIGS. 7A to 9D, an overlapping description thereof will beomitted.

According to embodiments of the present invention, at least followingeffects are obtained.

In one embodiment, a thin film transistor having a coplanar structure inwhich a resistance between an active layer and source and drainelectrodes can be minimized is disclosed. In one embodiment, a thin filmtransistor having improved characteristics is disclosed. In oneembodiment, a thin film transistor with an improved gate insulating filmis disclosed. In other embodiments, a method of manufacturing thedisclosed thin film transistors, and a display device including the thinfilm transistor can be provided.

The above-described effects according to the present invention are notintended to limit the contents used herein, and further effects may beencompassed in this specification.

It will be apparent to those skilled in the art that variousmodifications can be made to the above-described exemplary embodimentsof the present invention without departing from the spirit or scope ofthe invention. Thus, it is intended that the present invention coversall such modifications provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. An apparatus comprising: a gate electrode, asource electrode, and a drain electrode configured as a co-planarstructure for a thin film transistor; an oxide semiconductor layer,under said co-planar structure, configured to serve as a channel betweensaid source and drain electrodes; and a single layer of oxidizablematerial, on said oxide semiconductor layer and under said co-planarstructure, configured such that a first end region exhibits conductiveproperties and is in electrical contact with said source electrode, asecond end region exhibits conductive properties and is in electricalcontact with said drain electrode, and a central region between saidfirst and second end regions exhibits insulation properties as a resultof oxidation thereof to reduce electrical resistance between the channelof said oxide semiconductor layer and said source and drain electrodescompared to a co-planar TFT without said single layer of oxidizablematerial.
 2. The apparatus of claim 1, wherein a cross-sectional lengthof said central region is greater than or equal to a cross-sectionallength of said gate electrode.
 3. The apparatus of claim 2, furthercomprising: an insulation layer, on said single layer of oxidizablematerial and under said gate electrode, configured to serve as gateinsulation and also providing additional electrical resistance reductiontogether with said central region of said single layer of oxidizablematerial.
 4. The apparatus of claim 3, wherein said insulation layer isconfigured with contact holes that allow a portion of said sourceelectrode and a portion of said drain electrode to contact with saidfirst end region and said second end region of said single layer ofoxidizable material, respectively.
 5. The apparatus of claim 4, whereinsaid single layer of oxidizable material has an electron mobility thatis higher than that of said oxide semiconductor layer made of afour-component metal oxide material.